The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.

Molybdenum Oxides – From Fundamentals to Functionality

The global photovoltaic (PV) market is dominated by crystalline silicon (c-Si) based technologies with heavily doped, directly metallized contacts. Recombination of photo-generated electrons and holes at the contact regions is increasingly constraining the power conversion efficiencies of these devices as other performance-limiting energy losses are overcome. To move forward, c-Si PV technologies must implement alternative contacting approaches. Passivating contacts, which incorporate thin films within the contact structure that simultaneously supress recombination and promote charge-carrier selectivity, are a promising next step for the mainstream c-Si PV industry. In this work, we review the fundamental physical processes governing contact formation in c-Si. In doing so we identify the role passivating contacts play in increasing c-Si solar cell efficiencies beyond the limitations imposed by heavy doping and direct metallization. Strategies towards the implementation of passivating contacts in industrial environments are discussed. The development of passivating contacts holds great potential for enhancing the power conversion efficiency of silicon photovoltaics. Here, De Wolf et al. review recent advances in material design and device architecture, and discuss technical challenges to industrial fabrication.

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Passivating contacts for crystalline silicon solar cells

https://iopscience.iop.org/article/10.1088/1361-6463/ab9c6a/pdf

The 2020 photovoltaic technologies roadmap

The photovoltaic industry is dominated by crystalline silicon solar cells. Although interdigitated back-contact cells have yielded the highest efficiency, both-sides-contacted cells are the preferred choice in industrial production due to their lower complexity. Here we show that omitting the layers at the front side that provide lateral charge carrier transport is the key to excellent optoelectrical properties for both-sides-contacted cells. This results in a conversion efficiency of 26.0%. In contrast to standard industrial cells with a front side p–n junction, this cell exhibits the p–n junction at the back surface in the form of a full-area polycrystalline silicon-based passivating contact. A detailed power-loss analysis reveals that this cell balances electron and hole transport losses as well as transport and recombination losses in general. A systematic simulation study led to some fundamental design rules for future >26% efficiency silicon solar cells and demonstrates the potential and the superiority of these back-junction solar cells. Front- and back-junction silicon photovoltaics dominate the market thanks to a lower manufacturing complexity compared with that of other device designs yet advances in efficiency remain elusive. Richter et al. now present an optimized design for the front and back junctions that leads to a 26.0%-efficient cell.

Design rules for high-efficiency both-sides-contacted silicon solar cells with balanced charge carrier transport and recombination losses

23.5%-efficient silicon heterojunction silicon solar cell using molybdenum oxide as hole-selective contact

24.58% total area efficiency of screen-printed, large area industrial silicon solar cells with the tunnel oxide passivated contacts (i-TOPCon) design

Mass production of industrial tunnel oxide passivated contacts (i‐TOPCon) silicon solar cells with average efficiency over 23% and modules over 345 W

In this study, we present novel multilayer back contact (MLBC) solar cells employing V2Ox (8 nm)/metal/V2Ox (8 nm) (VMV) multilayers as dopant-free hole-selective contacts deposited using a thermal evaporation process at low temperature. The optimized V2Ox films have a high work function and reduced O-deficiency in-gap state energy owing to the introduction of a carefully controlled O2 partial pressure during the evaporation process. The contact resistivities of VMV (12 nm Ag) and VMV (4 nm Au) contacts with n-Si are 1.58 mΩ cm2 and 0.04 mΩ cm2, respectively, which are less than that of a 16 nm V2Ox/n-Si contact. Interestingly, VMV (Au)/n-Si MLBC solar cells demonstrate improved charge carrier transport, leading to an induced p–n junction. Moreover, the dominant interfacial charge carrier transport properties of MLBC solar cells with VMV (Ag)/n-Si and VMV (Au)/n-Si contacts correspond with the diffusion–recombination model, whereas, those of MLBC solar cells with V2Ox/n-Si and VMV (Ca)/n-Si contacts correspond with the multi-tunneling capture emission model at a high-forward-bias voltage. The use of VMV (4 nm Au) as an emitter achieves an efficiency of 19.02% for this type of MLBC solar cell, which is greater than that of V2Ox/n-Si solar cells (17.58%). This work has important implications for enabling the fabrication of low-performance dopant-free back contact solar cells with high stability using a simple fabrication process.

/pdf/dopant-free-multilayer-back-contact-silicon-solar-cells-50wtp6g04m.pdf

Dopant-free multilayer back contact silicon solar cells employing V2Ox/metal/V2Ox as an emitter

A high recombination rate and high thermal budget for aluminum (Al) back surface field are found in the industrial p-type silicon solar cells. Direct metallization on lightly doped p-type silicon, however, exhibits a large Schottky barrier for the holes on the silicon surface because of Fermi-level pinning effect. As a result, low-temperature-deposited, dopant-free chromium trioxide (CrOx, x < 3) with high stability and high performance is first applied in a p-type silicon solar cell as a hole-selective contact at the rear surface. By using 4 nm CrOx between the p-type silicon and Ag, we achieve a reduction of the contact resistivity for the contact of Ag directly on p-type silicon. For further improvement, we utilize a CrOx (2 nm)/Ag (30 nm)/CrOx (2 nm) multilayer film on the contact between Ag and p-type crystalline silicon (c-Si) to achieve a lower contact resistance (40 mΩ·cm2). The low-resistivity Ohmic contact is attributed to the high work function of the uniform CrOx film and the depinning of the ...

Wenjie Lin

Papers

24.58% total area efficiency of screen-printed, large area industrial silicon solar cells with the tunnel oxide passivated contacts (i-TOPCon) design

Mass production of industrial tunnel oxide passivated contacts (i‐TOPCon) silicon solar cells with average efficiency over 23% and modules over 345 W

Dopant-free multilayer back contact silicon solar cells employing V2Ox/metal/V2Ox as an emitter

Chromium Trioxide Hole-Selective Heterocontacts for Silicon Solar Cells

Multilayer MoOx/Ag/MoOx emitters in dopant-free silicon solar cells